U.S. patent application Ser. No. 08/077,014 filed on Jun. 14, 1993, incorporated herein by reference discloses a method for injecting a non-cryogenic inert gas such as nitrogen gas in the downhole region during drilling operations, to remove drill cuttings. This method presents advantages over downhole drilling using combustible gases such as air and cryogenic fluids such as liquid nitrogen.
Drilling and post-drilling operations efficiently establish a well, cement or secure casings or other tubular members within the well and remove the desirable payloads (e.g. gas and/or oil) from the well or directly from the reservoir containing the same. Methods of performing these operations are well-known.
Generally, the drilled wells are provided with tubular casings which secure the perimeter of the wellbore. Sometimes multiple casings (intermediates) are secured from the surface of the well to lower downhole locations. Other types of casings, called liners, are sometimes used to extend from the lowermost casing into the lowermost portion of the wellbore. Drilling fluids, such as drilling mud, are often used when large flows of water are present in the well. The drilling mud is circulated down the drill string, through the drill bit, and up the annular region between the drill string and the wellbore or casing to the surface. Gas may be injected in the downhole region to provide faster drilling rates when substantial amounts of water are not present in the well.
Air has been used as the principal downhole drilling fluid for low water content drilling. The air can be combined with a surfactant, foaming agent, water and/or mud for different applications. The primary advantages of straight air drilling are greatly increased penetration rates, greater bit footage and fewer downhole drilling problems.
Downhole drilling with air, however, does have a number of disadvantages, one of the most important of which is the occurrence of downhole explosions or fire due to the presence of high levels of oxygen in air. Efforts have been made to reduce the hazards of air drilling by lowering the temperature of the air or by replacing air with an inert gas. U.S. Ser. No. 08/077,014 discusses prior art efforts to solve the problem and discloses the advantages of using non-cryogenic inert gases (e.g. nitrogen) for this purpose.
There are other significant problems encountered in drilling and post-drilling operations. When a drilling fluid (e.g. drilling mud including optional chemicals and additives) is introduced into the downhole region, the weight of the drilling fluid creates a hydrostatic pressure proportional to the density of the fluid. The deeper the well, the greater the hydrostatic head pressure developed by the column of the drilling fluid.
The weight of the drilling fluid can be adjusted at the surface by changing the mud weight, or changing to a more or less dense drilling fluid. The drilling fluid can be lightened by co-mingling the drilling fluid with a lower density fluid such as a gas. Nitrogen gas is advantageous for this purpose because it is inert and non-corrosive.
In drilling operations, the formation pressure of the reservoir (i.e. the pressure exerted by the gas and/or oil) will vary throughout the downhole region. When the formation pressure is equal to the hydrostatic pressure of the drilling fluid, the fluid system is said to be balanced. If the formation pressure is less than the hydrostatic pressure of the drilling fluid, the system is overbalanced. Greater formation pressure than hydrostatic pressure results in an underbalanced system.
By maintaining an underbalanced system (i.e. the formation pressure exceeds the hydrostatic pressure of the drilling fluid), the formation pressure causes a net flow of the gas and/or oil into the wellbore. The density of the drilling mud must often be reduced to generate an underbalanced drilling condition. Air has been used to reduce the density of the drilling mud. However, under some circumstances, the presence of combustible air in the downhole region can create explosive conditions.
Another problem associated with downhole drilling relates to the installation of the casings and liners. Quite often the casings rub against the sides of the wellbore which makes installation difficult and can cause damage to the casing and/or the wellbore or formation of interest. The drill string, as well as subsequent casings or liners, are often filled with a drilling fluid and can become stuck in the downhole region, particularly when at least a portion of the downhole region of the well extends horizontally. In addition, the cementing of the casings within the downhole region is difficult because the cement has limited flexibility with regard to flow properties and weight distribution.
Post drilling operations also suffer from a number of difficulties. The removal of gas and/or oil from the downhole region presents several problems to drillers. First, gas and/or oil removal is inhibited by the presence of water and debris in the well. Second, the withdrawal of the gas and/or oil from deep wells requires high pressure equipment to displace the heavy well fluids from the well. Third, the permeability of the downhole region of the well often decreases over time thereby decreasing the rate at which gas and/or oil enter the production string from the reservoir. Fourth, gas and/or oil production depend on the pressure on the fluids within the reservoir. As the pressure decreases (depletes), production will decrease. Quite often production will cease from the lack of formation pressure even when significant amounts of gas and/or oil remain in the reservoir.
It would be a significant advance in the art of drilling for gas, oil and geothermal wells if the drilling and post drilling operations could be improved and particularly if an inert gas, typically a nitrogen rich gas, could be conveniently and efficiently supplied to the downhole region of the well and/or reservoir to eliminate or at least reduce the aforementioned problems.